1. Field of the Invention
The present invention relates to electro-optic semiconductor devices which incorporate multiple active and passive component parts and methods for making the same. In particular the present invention relates to semiconductor lasers and detectors with integrated semiconductor waveguides and extended cavity semiconductor lasers and methods for making the same.
2. Discussion of Prior Art
Electro-optic semiconductor devices are used in the quickly developing field of high speed analogue and digital optical signal processing circuitry which have applications in high speed, wide-bandwidth optomicrowave and optical transmission techniques particularly in the field of telecommunications.
Electro-optic semiconductor components, such as semiconductor lasers and waveguides, are currently grown separately in different growth processes to generate different component structures on separate crystal substrates. Subsequently, the separate component structures are assembled into a single subsystem. There are significant losses at interfaces between the separate component structures which lead to high input power requirements. Also, problems of accurately optically aligning the separate component structures leads to high complexity and cost.
Alternatively, electro-optic semiconductor components, such as semiconductor lasers and waveguides are grown with quantum well layers, which extend over the whole area of crystal growth. Selected areas of these quantum well crystal growth layers are subsequently destroyed by the process of quantum well intermixing in which the selected parts of the quantum well layers are caused to diffuse into surrounding crystal growth layers. Unfortunately, the quantum well diffusion method only allows device to be grown within a passive waveguide. Also, the diffuse material can change the properties of the surrounding crystal growth layers, for example, passive waveguide core layers, in an undesirable manner. The diffusion process is significantly alloy dependent, ie. what works for a GaAlAs structure does not work for a InGaP structure. This places a constraint on the type of materials that can be used in the quantum well layers. Also, this method of crystal growth requires a post processing quantum well intermixing step to destroy the selected areas of the layers of quantum wells, which adds expense.
A further method of growing layers of quantum wells within a passive waveguide core is to grow the layers of quantum wells over the whole area of crystal growth and then stop the crystal growth and selectively etch away unwanted areas of the layers of quantum wells. After the etching process a passive waveguide layer is grown over the remaining areas of quantum well layers. The problem with this process is that the growth process has to be interrupted to perform at least one etching process. This adds complexity and cost to the growth process and can reduce yield.
A method and device are described in U.S. Pat. No. 5,418,183 in which the device is made using selective area epitaxy in which a mask is deposited on the partly grown device and the mask is defined by an etching process. This is followed by further epitaxial growth in the areas exposed by the mask and then by removal of the mask by etching. The structure and thus the resulting characteristics of the epitaxial growth in the areas exposed by the mask are critically dependent on the dimensions of the deposited mask. Also, the removal of the partly grown structure from the epitaxial growth chamber for the two etching stages and the processes involved in the etching stages can introduce contamination into the resulting device. Accordingly, the method described is relatively complex and can have associated low yields.
The present invention aims to overcome at least some of the above mentioned problems by providing electo-optic semiconductor devices which are cheap, have low losses and so reduced power requirements and are robust.
Therefore, according to the present invention there is provided an electro-optic semiconductor device comprising a semiconductor waveguide with a core region within which is located at least one active area wherein the core of the waveguide outside of the active area is not contaminated with diffuse active area material and the active area and waveguide are monolithic and are grown in an additive growth process.
Also, according to the present invention, there is provided an electro-optic semiconductor device comprising a semiconductor waveguide with a core region within which is located at least one active area wherein the profile of the core region follows the profile of the active area, and the active area and waveguide are monolithic and are grown in an additive crystal growth process.
The electro-optic semi-conductor device according to the present invention is monolithic and so is a structure composed of a single continuous crystalline mass of material.
The term additive growth process is used to define a growth process in which a monolithic crystal structure is grown without any intermediate stages in which material is removed from the crystal previously grown. For example, it excludes growth processes which have an intermediate etching stage. Clearly, an additive growth process will generally be simpler and so will provide a cheaper way of making a monolithic crystal substrate comprising an active area located within a passive waveguide region.
The present invention combines this advantage with the advantage that no post processing stage is require to destroy selected parts of the active areas. Also, as the resulting device is monolithic there are very low coupling losses between the active area and the waveguide.
In post processing steps the active areas are configured as electro-optic components, such as lasers and detectors. Integration of at least one active area in the core of a waveguide enables optical connection between a plurality of electro-optical components, configured from the active areas, within a single crystal structure. Subsequent device configuration uses conventional and established processing technology to configure the active areas into electro-optic components. This monolithic integration will allow repeatable and reliable fabrication on a single substrate with low cost if high volumes are achieved. The resultant devices are simple, compact and robust because the entire device is a single crystalline structure. Optical alignment of, for example, a laser/waveguide interface requires no mechanical alignment, and so good optical alignment is achieved cheaply and without adding complexity.
Preferably the active area is located between two adjacent growth layers of the core of the waveguide. The growth method for this structure is very simple and enables multiple active areas to be grown in a single epitaxial growth which does not require multistage processing or regrowth.
An active area preferably comprises at least one quantum well layer or thin bulk layer having a smaller bandgap than the core of the waveguide. The parts of the resulting single crystal structure outside an active area become the passive optical interconnections defined by the waveguide whereas the regions with quantum wells become, for example, active laser or detector devices on subsequent processing. This is because the waveguide, having a wider bandgap is transparent to the light generated or detected by the active area.
It is preferable that the bandgap in an active area is less than the bandgap in a transition region between that active area and the core of the waveguide. This reduces losses due to band gap absorption outside of the active area.
The quantum well layers may be configured to act as a laser by post growth processing in which a laser stripe and an electrode are added. Alternatively, the quantum well layers may be configured to act as a detector by virtue of their absorption of light when they are reverse biased.
Preferably, the waveguide and active areas are grown using chemical beam epitaxy (CBE). CBE allows a simple mechanical shadow mask to expose well defined areas over which active areas can be grown. This is because the method of CBE allows a shadow mask to be selected of a material with which the CBE chemicals will not react strongly and so crystal deposits over the shadow mask are reduced. This combined with the area definition achievable in a molecular beam process such as CBE and the fact that CBE is a UHV (ultra high vacuum) technique enables a mechanical shadow mask technique to give good edge definition to active areas.
Alternative epitaxy methods which are suitable are Molecular Beam Epitaxy (MBE) and Metal Organic Vapour Phase Epitaxy MOVPE.
The waveguide and active area may comprise a III-V semiconductor composition which may be Indium Phosphide based which can form the basis for active electo-optical components operating at wavelengths, typically of, 1.3 and 1.5 microns.
Alternatively, the waveguide and active area may comprise a III-IV semiconductor composition which may be InAs, GaSb, or InSb based for electro-optic components operating at wavelengths from 1 micron to greater than 8 microns or GaAs based operating at visible to UV wavelengths.
In one embodiment according to the present invention the III-V semiconductor composition is Gallium Arsenide based which can form the basis for active electro-optic and optical components operating at wavelengths of 860 nm. These short operating wavelengths result in compact components, such as lasers and detectors, which are generally stable as a function of temperature, are more efficient and exhibit low noise. Furthermore, GaAs based processing technology is relatively advanced compared to other III-V semiconductor devices.
Where a GaAs based III-V semiconductor device working a 860 nm is required the waveguide can comprise Aluminium Gallium Arsenide and the active area can contain at least one layer of Gallium Arsenide and the core of the waveguide comprises typically 20% to 40% Aluminium and upper and lower cladding layers of the waveguide can contain typically 40% to 80% Aluminium. Furthmore, one of the cladding layers may be n-type doped and the other of the cladding layer may be p-type doped.
Preferably, a layer of GaAs which is at much thinner than a GaAs layer in the active area is located in the core of the waveguide beyond the active area and in a common plane with the active area. This prevents lengthy exposure of an AlGaAs substrate layer during crystal growth as is explained further below.
The device according to the present invention may be an extended cavity laser comprising at least one quantum well layer located within a core of a waveguide. The extent of the laser cavity may be defined by the natural cleavage planes of the monolithic crystal from which the device is formed.
The device according to the present invention may have a plurality of active areas within the core region. This enables a plurality of electro-optical devices to be integrated in a single monolithic crystalline substrate.
According to a second aspect of the present invention there is provided a method of making an electro-optic semiconductor device, comprising the steps of;
growing a first part of a core layer of a semiconductor waveguide,
selectively growing at least one active layer over a partial area of the first part of the core layer, and
growing a second part of the core layer of the semiconductor waveguide over the first part and the active layer,
wherein the method comprises an additive crystal growth process.
The selective growth of an active layer may conveniently comprise the steps of;
selectively covering a portion of the first part of the core layer to expose a partial area of the first part of the core layer,
growing at least one active layer on the exposed partial area, and
uncovering the first part of the core layer.
The method according to the present invention enables the creation and integration of optical and electro-optic semiconductor devices (active and passive) on a single semiconductor wafer. Monolithic and planar integration of optical semiconductor devices is enabled on smaller lateral scales with less processing than previously possible by integration of active region in a waveguide layer at growth stage. Also, the need for etching and regrowth is removed. Performance and robustness are thus improved and cost, size, power requirement and weight reduced.
Waveguide cladding layers may be grown above and below the core layer and the active layer may comprise at least one quantum well layer having a smaller bandgap than the core of the waveguide.
As discussed above it is preferred that at least the core layer and active layer are grown using chemical beam epitaxy (CBE).
In a GaAs based semiconductor device with a layered structure described above, in order to reduce impurities in the growth crystal and so improve the optical characteristics of the resulting active and passive components, the growth is preferably conducted at a temperature of 400 to 700xc2x0 C.
The first part of the core of the waveguide is preferably selectively covered by a mechanical shadow mask which may be made of silicon coated with silicon oxide, silicon dioxide or silicon nitride in order to reduce the amount of crystal growth on the mask surface. To accurately define the edges of the active layer it is preferred that the edges of the shadow mask defining an aperture which exposes an area of the first part of the core of the waveguide are tapered so that the edges adjacent to the area of the first part of the core of the waveguide form the thin end of the taper.
The method according to the present invention may be used to selectively grow a plurality of active layers over different partial areas of the first part of the core layer. This enables a plurality of active electro-optical devices, such as lasers and detectors to be integrated in a single monolithic crystalline substrate.